Regulated membrane and protein transport is a central component of a multitude of biological processes and is critically important in human heath. For example, transport defects are implicated in hypercholesterolemia, Huntington's disease, and myeloid leukemia. Thus, elaborating molecular mechanisms of membrane trafficking are profoundly important to both basic and applied research. Our interests are in exploiting advantageous features of C. elegans and mammalian cells to investigate one of the least understood steps in endocytic traffic-the return of internalized molecules to the cell surface via recycling endosomes. Surprisingly little is known of the molecules that mediate and regulate this step of recycling. Initial studies indicate this process is highly conserved in metazoans, and may be distinct from related processes in microbes like yeast. Using a genetic approach in C. elegans, we have identified novel proteins that are critical for the recycling process in vivo. One of these, RME-1, is a peripheral membrane protein of the recycling endosome that regulates export from the recycling endosome in a process thought to utilize membrane tubules. RME-1 appears highly conserved between nematodes and mammals in both structure and basic function and is one of the first proteins clearly implicated in this specific recycling step. Mutations in a second protein, GUM-l, cause the same biological defects as mutations in RME-I. Furthermore, GUM-1 is required for the localization of RME-1 to endosomes. The molecular identity of the GUM-1 protein and the mechanism by which it regulates RME-1 localization are currently unknown. We plan to decipher the biology of endocytic recycling by conducting an in-depth multi-pronged analysis of RME-1 and GUM-1 and by applying novel approaches toward the identification of additional proteins that act with RME-1 and GUM-1 in the process of recycling. Our specific aims are: l) To test key working hypotheses that RME-1 is regulated by nucleotide binding and membrane association, and that RME-1 participates in membrane tubule formation. 2) To characterize GUM-l, a novel regulator of endocytic recycling in C. elegans and mammals. 3) To identify additional mediators and regulators of endocytic recycling in C. elegans. By combining the power of C. elegans genetics and mamalian cell biology this work will significantly advance our understanding of .the mechanisms determining a key metazoan membrane transport process.